Immunologic techniques were applied to the study of EBV oncogenesis in marmosets. Tumors were produced in a white-lipped and common marmoset. A specific cellular immune response to EBV, the appearance of serum blocking factor, and elevated EBV antibodies were associated with the appearance of tumor in the white-lipped marmoset. Four rabbits inoculated with Herpesvirus saimiri developed malignant lymphoma within two months of inoculation.